Combined with measures against global warming, a departure from dependency of energy on crude oil has become a world-wide important issue, and efforts to yield practically usable fuel cells using hydrogen gas as an energy source are stepping up not only in European industrialized countries, where efforts for environmental protection have preceded, but also in the United states of America, and Japan and other Asian countries.
A number of studies on methods of producing hydrogen gas used as a fuel for fuel cells has been pursued. Production methods that are most inexpensive and most practicable at present are methods of producing hydrogen gas by reforming, for example, natural gas, liquid petroleum gas (LPG), kerosene, gasoline, methanol, or dimethyl ether as a raw material. Steam reforming is most generally used in methods of producing hydrogen gas by reforming such raw materials, such as a process of producing hydrogen by reforming natural gas. The natural gas mainly contains methane (CH4), and hydrogen is generated by steam reforming through two-stage reactions as follows:
(1) Reforming ReactionCH4+H2O→CO+3H2 
(2) Shift ReactionCO+H2O→CO2+H2 
If these reactions ideally proceed, a product contains H2 and CO2 alone. However, a gas after the reforming reaction and shift reaction (hereinafter referred to as “reformed gas”) contains steam (H2O), unreacted methane (CH4), carbon monoxide (CO), and carbon dioxide (CO2) in addition to hydrogen (H2), because excessive steam is used in practice from the viewpoint of preventing the formation of carbon due to coking of methane. Fuel hydrogen for fuel cell powered vehicles generally requires a high hydrogen purity of about “five nines” (99.999 percent by volume (hereinafter “percent by volume” is simply referred to as “%”)) or more. Particularly, it is believed that a carbon monoxide concentration of the fuel hydrogen must be lowered to 10 ppm or less from the viewpoint of preventing deterioration of platinum (Pt) due to poisoning, which platinum is used as an electrode catalyst in solid polymer fuel cells (proton exchange membrane fuel cells); and the carbon monoxide concentration must further be lowered to about 0.2 ppm or less in consideration of durability of fuel cells.
The following two processes are representative processes of purifying hydrogen from a reformed gas:
(1) a selective oxidation catalyst process; and
(2) a hydrogen pressure swing adsorption (hydrogen PSA) process
The selective oxidation catalyst process (1) is a technique which has been developed mainly aiming at stationary fuel cells including domestic-use fuel cells. According to this technique, a reformed gas is added with air or oxygen, CO gas in the reformed gas is selectively oxidized using a catalyst to remove carbon monoxide in the form of CO2 from the reformed gas, and the fuel cell is thus prevented from poisoning by carbon monoxide. This technique is characterized in that it is a process carried out under normal pressure and that it can use a small-sized apparatus because it can be carried out at a relatively high superficial velocity (SV). However, it is difficult to use this technique as a purification technique to yield such a high-purity hydrogen as to be required in on-vehicle fuel cells, because the technique is not a technique of removing CO2, H2O, and CH4 as other impurities than carbon monoxide.
On the other hand, the hydrogen PSA process (2) is a process of removing all CO2, CH4, H2O, and CO from a reformed gas while carrying out pressure swing and using two or more adsorbents such as zeolite, a carbon molecular sieves, and alumina. In addition, hydrogen to be supplied to transportation fuel cells (automotive fuel cells) must be free from not only carbon monoxide but also other impurities, and the hydrogen PSA process is generally employed for producing fuel hydrogen by reforming fossil fuel in hydrogen supply stations.
When hydrogen is purified according to the hydrogen PSA process, impurities other than hydrogen are removed by adsorption under high-pressure to recover a product hydrogen. A PSA adsorbent adsorbing CO, CH4, H2O, and CO2 as impurities is allowed to desorb the adsorbed CO, CH4, H2O, and CO2 by operations of reducing the pressure from a high pressure to normal pressure and washing the adsorbent with the product hydrogen. Thus, the adsorbent is regenerated. An adsorption tower in which the adsorbent has been regenerated is again raised in pressure, supplied with a reformed gas, and subjected to another purification operation to yield a product hydrogen.
As problems in the hydrogen PSA process, the known hydrogen PSA process requires very large-sized facilities (very large-sized adsorption towers), because it is difficult to remove carbon monoxide which is contained in a crude hydrogen in a content of up to about 1%, and a large quantity of adsorbents is required. In addition, cost for purifying hydrogen is increased, because the recovery of the product hydrogen is not sufficiently high.
A variety of techniques has been developed for solving these problems. For example, Patent Document 1 discloses a technique of improving the recovery of hydrogen gas from 70% in a known technique up to 76% by a process of carrying out the step of washing an adsorption tower after the adsorption of impurities until at least part of a cleaning gas which has been introduced into the tower to be cleaned is delivered from the tower.
Patent Document 2 discloses a technique of improving the hydrogen gas recovery to 76% by using, as a cleaning gas, a gas in an adsorption tower after the completion of an adsorbing step and setting the amount of the cleaning gas at 2 to 7 times as much as that of the packing volume of the adsorbent. In addition, Patent Document 3 discloses a technique of downsizing hydrogen PSA facilities and improving the hydrogen recovery to 74% by single use of, as an adsorbent, zeolite having a faujasite structure with a silicon/aluminium ratio of 1 to 1.5 and having a lithium ion exchange rate of 95% or more.
According to these techniques, however, all impurity gases including carbon monoxide in hydrogen are removed by the hydrogen PSA process, and adsorbents have insufficient adsorption capacities of CO gas. Thus, there is limitation in downsizing of facilities. In addition, the hydrogen recovery is still insufficient, although improvements by the various techniques have been studied.
As another technique for contributing to downsizing of a hydrogen PSA system, there is studied a technique in which carbon monoxide is not removed directly by hydrogen PSA but removed by oxidizing carbon monoxide in a reformed gas with a selective oxidation catalyst into CO2 and subjecting the resulting reformed gas to hydrogen PSA facilities to thereby remove CO2, CH4, and H2O in hydrogen (Non-patent Document 1). Although this technique is effective for downsizing the hydrogen PSA system, it causes a lowered hydrogen recovery when considered as a whole system, because excessive oxygen is introduced in selective oxidation of carbon monoxide, and oxygen which has not reacted with carbon monoxide reacts with hydrogen to thereby consume hydrogen.
Non-patent Document 1: New Energy and Industrial Technology Development Organization (NEDO) Report for Fiscal Year 2001, Development of Hydrogen Production Technologies According to New PSA System, 2002
Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-177726 (e.g., claims)
Patent Document 2: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-191923 (e.g., claims)
Patent Document 3: Japanese Unexamined Patent Application Publication (JP-A) No. 2002-191924 (e.g., claims)